B.E. to Msc?

i am currently doing my B.E. Mechanical Engineering. I am very interested in doing my post graduation MSc in a foreign university in US like MIT, Caltech, Cornell etc (Even others would be wonderful but I cant recollect any as of now:) ). I have very good grades. I have been regularly securing University Rank. Plus, as an extra curricular activity, I have won many chess championships in the district and college. The area I want to do my MSc is in Astrophysics/Quantum Mechanics. Though I have not been directly involved with the subject in my course, I have been working on it otherwise and have even published a couple of papers on Relativity. Plus, I have taken part in many pure science based seminars and have been involved in projects on QM, The Standard Model etc with many professors. Despite having a BE in Mechanical Engineering as the backgrond as mentioned above, what are my chances of doing an MSc in astrophysics, QM, Relativity or similar pure physics subjects?